Jagdish Dukre

OPTIC NERVE HYPOPLASIA

Optic nerve hypoplasia (ONH) is the most common optic disc anomaly and is the third leading cause of blindness in children in the western world after cerebral damage and retinopathy of prematurity.

Risk factors for ONH include

young maternal age,

maternal smoking,

preterm birth and its complications.

Clinical features An abnormally small optic

nerve head.

Double-ring sign:

The optic nerve is pale surrounded by a yellowish peripapillary ring of sclera and an outer concentric ring of hypopigmentation.

outer ring of normal junction between sclera and lamina cribrosa

inner ring denoting extension of retina and RPE over lamina cribrosa

A disc to centre of macula (DM): mean disc diameter (DD) ratio

DM:DD ratio of

3.00 should lead to serious consideration of the diagnosis and a value of

4.00 probably accords a definitive diagnosis.

The visual acuity is associated with the DM: DD ratio.

In one study of 19 children with ONH, all eyes with a DM: DD ratio of more than 3 had reduced visual acuities but all those with a ratio of less than 3 had normal acuities.

Histopathologically,

optic nerve hypoplasia has subnormal number of axons with normal mesodermal elements and glialsupporting tissue.

A reduction in the diameter of the hypoplastic optic nerve and chiasm is demonstrated reliably by MRI, which establishes the presumptive diagnosis of optic nerve hypoplasia.

Visual acuity:

ranges from 20/20 to no light perception.

Visual acuity usually remains stable throughout life unless amblyopia develops in one eye or it is associated with suprasellar tumour where it can lead to acquired visual loss.

Optic nerve hypoplasia has recently been implicated in the pathogenesis of amblyopia although it is still not clear whether a small optic disc area is the cause for decreased vision or the associated hyperopia and anisometropia.

Visual field defects: the affected eye shows localized visual field defects.

Astigmatism: warrants attention to correction of refractive error in children.

Associations: Magnetic resonance imaging has revealed coexistent CNS abnormalities with ONH.

1) Isolated ONH: A reduction in the diameter of the hypoplastic optic nerve and chiasm is demonstrated reliably by MRI, which establishes the presumptive diagnosis of optic nerve hypoplasia.2) Septo-optic dysplasia (de Morsier syndrome) constellation of optic nerve hypoplasia, absence of the septum pellucidum, and partial or complete agenesis of the corpus callosum.

sagittal MR shows absence of thecorpus callosum

3) Forebrain malformations, schizencephaly and periventricularleukomalacia, absence of the pituitary infundibulum with or without posterior pituitary ectopia (can present in upto 74%)

Cerebral hemispheric abnormalities indicate that neurodevelopmental deficits are likely. Absence of the pituitary infundibulum with an ectopic posterior pituitary gland indicates congenital hypopituitarismand warrants endocrinologic evaluation in children. Growth hormone deficiency is most common which usually presents by 3-4 yrs of age

MORNING GLORY DISC ANOMALY The morning glory disc anomaly is a

congenital excavation of posterior globe that involves the optic disc.

The term reflects the morphological similarity to the flower of the morning glory plant.

Morning glory syndrome is ostensibly a sporadic condition.

It usually occurs as a unilateral condition, though bilateral lesions have been reported.

Morning glory discs are more common in females (2:1)

Pathogenesis

The pathogenesis of the condition is unknown.

One hypothesis argues that the condition results from failure of closure of the foetal fissure and that it is a variant of optic nerve coloboma.

Alternatively, a primary mesenchymal abnormality has been postulated on the basis of the glial tuft, the scleral and vascular abnormalities.

Clinical featuresOphthalmosopic appearance:

It is characterized by

funnel-shaped and enlarged dysplastic optic disc with white tissue, and retinal vessels arising from the periphery of the disc (spoke wheel pattern) and running an abnormally straight course over the peripapillaryretina .

Visual acuity: visual acuity is often very low, being in the range of counting fingers to 6/60, but it can be associated with no perception of light.

Amblyopia: variable depth based on the extent of the alteration of the nerve fiber layer and difficult to predict on the basis of disc appearance

AssociationsWith rare exceptions, the Morning glory disc anomaly is not part of a multisystem genetic disorder.Ocular anomaliesRetinal detachment, both serous and rhegmatogenous, occurs in about one- third (26-38%) cases of MGDA.StrabismusIntracranial disordersBasal encephalocele: Children who have this occult basal encephalocele have characteristic facial features including hypertelorism, depressed nasal bridge, midline upper lip notch and sometimes midline cleft in soft palate.Agenesis of corpus callosumAbsence of chiasmPanhypopituitarismMost affected children have no overt intellectual or neurologic deficits.

(A) Morning glory optic disc anomaly; (B) hypertelorism and flat nasalbridge – note bilateral iris colobomas;

OPTIC DISC COLOBOMA Optic disc coloboma can be present in one or both

eyes.

They may arise sporadically or be inherited in autosomal dominant fashion.

It has recently been shown to be associated with PAX2 gene mutations as part of the renal-colobomasyndrome.

Pathogenesis: It is caused by an incomplete or abnormal apposition of the proximal ends of the embryonic fissure.

Clinical features Optic disc coloboma is

typically an enlarged disc with sharply demarcated, focal, glistening bowl shaped excavation of the optic disc.

The coloboma occupies the lower part of the optic nerve head.

The neuro-retinal rim is absent inferiorly but is usually identifiable superiorly.

(A) Optic disc coloboma; (B) FA shows hypofluorescence of the cavity

Visual acuity: may be minimally or severely affected and difficult to predict based on the appearance.

Careful analysis of the photographic appearances of colobomata involving the optic nerve has shown that the only feature that relates to visual outcome is the degree of foveal involvement by the coloboma.

Significant refractive error and anisometropia are common and need to be dealt with optimal glass prescription.

AssociationsOptic nerve colobomas may be associated with the

following

Microphthalmos

Iris coloboma, lens coloboma and retinochoroidalcoloboma

Serous macular detachment: can be rhegmatogenous or non rhegmatogenous.

Systemic associations like :

CHARGE syndrome (coloboma, heart anomaly, choanalatresia, retardation, genital and ear anomalies)

Walker- Warburg syndrome

Goldenhar’s syndrome

It needs to be distinguished from the other congenital disc lesions, especially morning glory disc .

OPTIC PIT

Optic nerve pits are rare congenital anomalies that are part of a spectrum of congenital cavitary optic disc anomalies that may be associated with juxtapapillaryretinal detachments.

They occur in less than 1 in 10,000 patients seen in an ophthalmic setting and are bilateral in 10% to 15% of cases.

Pathogenesis These are formed through the herniation of dysplastic

retina into a collagen-lined pocket extending posteriorlythrough a defect in the lamina cribrosa.

They occur in locations unrelated to the embryonic fissure, commonly located at temporal aspect of disc

Clinical features An optic disc pit is a round or

oval, gray, white or yellowish depression in the optic nerve head.

Visual acuity is usually normal unless associated with serous macular detachment.

Visual field defects: paracentral arcuate scotomais the most common.

(A) Optic disc pit; (B) optic disc pit and macular detachment

Associations

Approximately one third to two thirds of patients with optic pits develop serous macular detachments.

These may occur during childhood or late in life but are most common between the ages of 20 and 40.

The fluid from optic pit initially produces an inner layer retinal separation (retinoschisis) that overlies posterior pole which then develops outer macular hole and leads to serous macular detachment

Various theories have been postulated for the source of the subretinal fluid such as

1) cerebrospinal fluid (CSF),

2) liquefied vitreous entering through the pit or through a macular hole, and

3) leakage from either choroidal vessels or permeable vessels in the pit.

Optic pits are not associated with brain malformations – so they do not warrant neuroimaging.

TreatmentThe treatment of serous macular detachment associated with an

optic disc pit is still controversial.

Observation: spontaneous resolution has been seen in 25% cases.

Laser photocoagulation: creating a barrier of chorioretinaladhesions at the temporal optic disc border but repeated treatments may be required.

Gas tamponade with laser photocoagulation: this induces pneumatic displacement of the outer layer detachment and improves central vision.

Pars plana vitrectomy (PPV) combined with laser photocoagulation and gas tamponade: currently this approach is considered to be more effective than laser photocoagulation or gas tamponade alone, particularly in eyes with severe visual loss

MEGALOPAPILLA Megalopapilla is a generic term that connotes an

abnormally large optic disc that lacks the inferior excavation of optic disc coloboma or the numerous anomalous features of the morning glory disc anomaly.

This condition is usually bilateral and often associated with a large cup-to-disc ratio.

Patients who have megalopapilla are often suspected to have glaucoma.

Unlike the situation in glaucoma, however, the optic cup is usually round or horizontally oval with no vertical notch or encroachment .

Visual acuity is generally normal in megalopapilla but may be mildly decreased in some cases.

Visual fields are usually normal except for an enlarged blind spot, which enables the examiner to rule out low-tension glaucoma or a compressive lesion.

Megalopapilla is only rarely associated with brain anomalies, and neuroimaging is not warranted unless midline facial anomalies are present.

CONGENITAL TILTED DISC SYNDROME The tilted disc syndrome is a nonhereditary bilateral

condition.

The superotemporal optic disc is elevated and the inferonasal disc is displaced posteriorly, which results in an optic disc of oval appearance with its long axis obliquely orientated.

This configuration is accompanied by situs inversus of the retinal vessels, congenital inferonasal conus, thinning of the inferonasal retinal pigment epithelium and choroid, and myopic astigmatism.

These features presumably result from a generalized ectasia of the inferonasal fundus that involves the corresponding sector of the optic disc.

(A) Tilted disc; (B) tilted disc and inferonasal chorioretinal thinning

Familiarity with this condition is important because affected patients may present with the suggestion of bitemporal hemianopias, which involve primarily the superotemporal quadrants.

However, these field defects, when observed carefully, do not respect the vertical meridian (as do chiasmallesions).

Furthermore, large and small isopters are fairly normal, but medium-sized isopters are constricted selectively because of the ectasia of the midperipheral fundus.

Repeated visual field tests after correcting for the myopic refractive error often eliminate the field defect, which confirms its refractive nature.

In some cases, retinal sensitivity is decreased in the area of the ectasia, which causes the defect to persist despite refractive correction.

Rare cases of the tilted disc syndrome have been documented in patients who have congenital suprasellartumors; neuroimaging is therefore warranted when the associated bitemporal hemianopia respects the vertical meridian.

CONGENITAL OPTIC DISC PIGMENTATION

Congenital optic disc pigmentation is a condition in which melanin anterior to or within the lamina cribrosa imparts a gray appearance to the disc.

True congenital optic disc pigmentation is extremely rare.

Congenital optic pigmentation is compatible with good visual acuity but may be associated with coexistent optic disc anomalies that decrease vision.

Most cases of gray optic discs are not caused by congenital optic disc pigmentation.

For reasons that are understood poorly, optic discs of infants who have delayed visual maturation or albinism and those of some normal neonates have a diffuse gray tint when viewed ophthalmoscopically.

In these disorders, the gray tint disappears within the first year of life without visible pigment migration.